Fixing error with unallowed commands

When running this to test the ssh connectivity, I found this bug:

```
-> % ssh git@gitlab ls -la
/home/git/gitlab-shell/lib/gitlab_shell.rb:102:in `join': can't convert nil into String (TypeError)
	from /home/git/gitlab-shell/lib/gitlab_shell.rb:102:in `escape_path'
	from /home/git/gitlab-shell/lib/gitlab_shell.rb:57:in `parse_cmd'
	from /home/git/gitlab-shell/lib/gitlab_shell.rb:22:in `exec'
	from /home/git/gitlab-shell/bin/gitlab-shell:16:in `<main>'
```

The expected behaviour would be :

```
-> % ssh git@gitlab ls -la
Not allowed command
```

Excuse me for my ruby style, I am not used to this language.